The present invention relates to an improved electroconductive composition which comprises antimony-containing tin oxide in which the tin oxide is predominately crystalline and the composition exists in a unique association with silica or a silica-containing material, e.g., a silicate. More particularly, the present invention relates to an improved electroconductive powder composition comprising tens of microns to sub-micron size particles having a thin surface layer of amorphous silica or silica-containing material, said material having a thin surface coating layer which comprises a network of antimony-containing tin oxide crystallites and to a process for preparing the composition.
U.S. Pat. Nos. 4,373,013 and 4,452,830 describe the preparation of an electroconductive powder having a structure comprising titanium oxide particles as nuclei with a coating of antimony-containing tin oxide on the surface of the titanium oxide particles. The powder is prepared by mixing an aqueous dispersion of titanium oxide particles with a solution containing a hydrolyzable tin salt and a hydrolyzable antimony salt. The titanium oxide particles become coated with antimony-containing tin oxide and can then be recovered by filtration.
"Journal of Materials Science", 21 (1986), pp. 2731-2734, describes the preparation of antimony-doped SnO.sub.2 films by thermal decomposition of tin 2-ethylhexanoate on glass substrates. Reagent grade tin 2-ethylhexanoate and antimony tributoxide were used as the sources of tin and antimony, respectively, and application of the film onto the substrate was accomplished by dipping the substrate into an alcoholic solution containing the organometallic compounds and then drying the applied solution. The substrate used was soda-lime glass which was previously coated with about a 30 nm layer of TiO.sub.2, SiO.sub.2 or SnO.sub.2 (with 8 wt % Sb) by thermal decomposition of organometallic compounds. The resistivity of the resulting film in which the substrate had a precoating of SiO.sub.2 was one-thirtieth of the resistivity of the antimony-doped tin oxide film on the uncoated glass substrate. For the range of films prepared, however, electrical properties were noted as being more or less poor compared with films obtained by other methods, such as, spraying or chemical vapor deposition.
Japanese Patent No. SHO 63[1988] 20342 describes a method of manufacturing fine electroconductive mica particles by coating them with a tin oxide/antimony oxide mixture. This coating is accomplished by treating the mica with tin tetrachloride, antimony trichloride, and a hydroxyl-containing, low-molecular-weight fatty acid.
Compositions which are capable of imparting electroconductive properties to thin films, such as, in polymer films, magnetic recording tapes, work surfaces and in paints, are not always economically attractive or reliable for a given application. Electroconductive compositions, e.g., powders, which are currently available for use as conductive pigments in paint, for example, suffer a variety of deficiencies. Carbon black may be used to impart conductivity, but this can limit the color of the paint to black, dark gray and closely related shades. Titanium dioxide powders, coated with antimony-doped tin oxide by methods of the prior art, normally require high pigment/binder ratios, e.g., 200/100, in order to achieve minimum acceptable surface conductivity. Such a high pigment loading is expensive and can limit the color range and transparency of the resulting paint to very light shades and pastels. A simple powder of antimony-doped tin oxide may be used, but cost and color limitations can be unfavorable.
Mica powders can be made conductive by coating the particles directly with antimony-doped tin oxide, but the preparation of such powders can be expensive and difficult because of the poor affinity of tin and antimony intermediates for the surface of the mica. Organic complexing agents and/or organic solvents are typically used to facilitate the reaction of tin and antimony intermediates with the mica surface. Even with these additives, a significant portion of the tin and antimony remain in solution or as free particles. This reduces the effective conductivity of the powder and increases the cost, since a significant amount of the tin and antimony values are lost when the coated particles are recovered from the reaction medium. In addition, the tin and antimony values remaining in solution must be removed before the waste solution which remains is discharged. Finally, the antimony-doped tin oxide layer has been found to bond poorly to the mica and may delaminate during subsequent processing, such as during milling or during incorporation into a polymer vehicle, e.g., a paint formulation or polyester film.